The word telephoto has come to mean long focal length, however that is not the original (nor correct) meaning of the word. But before I try to clarify the definitions, let me back up a few steps to discuss some theory, particularly as it relates to homemade lenses.

Focal length and back focal length

Hopefully, you've read my Homemade Lens Primer, in which I discuss the topics of focal length and back focal length. Photographers intuitively understand focal length. For a given object, at a given distance, focal length determines the size of the image of the object on the film or sensor. The longer the focal length, the larger the image size.

Back focal length is the distance from the rear element of the lens to the film/sensor when the with the lens focused at infinity. For a single element lens, the focal length and back focal length will be the same. However, when you combine two or more lenses, things get much more interesting.

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